1. Field of Invention
This invention relates to ultrasonic piezoelectric transducer driving systems for use in ultrasonic tools which include a piezoelectric transducer to convert an ultrasonic electric signal into an electric mechanical vibration and especially to ultrasonic tools which require high-performance and safe and reliable operation.
2. Prior Art
When power is supplied to an ultrasonic piezoelectric transducer, the resonance frequency of the transducer varies according to the mechanical load on the transducer, variations and the temperature of the transducer, etc. As a result, the driving frequency deviates from the resonance frequency of the transducer to thereby lead to a drop in the electro-mechanical transducing efficiency of the transducer. This tendency is especially noticeable in high-Q piezoelectric transducers which have a high transducing efficiency. In these cases, a slight deviation of the driving frequency from the resonance frequency causes the electro-mechanical transducing efficiency to drop substantially to a point where practical use of the transducer becomes impossible. Accordingly, in cases where a high-Q piezoelectric transducer with a high electro-mechanical transducing is utilized, an automatic frequency tracking system which causes the driving frequency to vary along with the resonance frequency of the transducer is essential. While such frequency tracking systems exist in the prior art, such systems have certain disadvantages. In particular, such systems usually apply a high driving power to the transducer without providing electrical insulation between the transducer and the driving circuit. Accordingly, the danger of electrical shock is significant. In addition, such driving circuits also use conventional amplifier circuits to amplify the ultrasonic electrical signal and such amplifier circuits are not efficient.
Furthermore, in the prior art there are several types of ultrasonic transducer driving systems utilizing a phase lock loop. Such systems are described in the U.S. Pat. No. 3,931,533 issued to Frank A. Raso, U.S. Pat. No. 3,975,650 issued to Stephen C. Payre and U.S. Pat. No. 3,447,051 isued to John G. Atwood. However, the above described systems provide no protection against electrical shock hazards as is required in medical instruments and does not maintain a high level of performance. The appearance of the PZT type piezoelectric elements has caused a great improvement in the electro-mechanical transducing efficiency. In the case of such piezoelectric transducers (even the voltage driven type), however, an attempt to supply sufficient power results in a high service voltage. Accordingly, such transducers cannot be used in medical applications without taking sufficient protective measures against electrical shock.